Acoustic cloak to hide ships from sonar

A new design for sound cloaking brings us one step closer to hiding ships from sonar and designing new kinds of concert halls.

The new sound shield is made of a stack of plastic sheets that have an intricate pattern of holes poked through them. When sound waves encounter the structure, they get re-routed in a specific path through the maze of holes and plastic plates. The result: instead of bouncing off the structure, as they would if they encountered any other obstacle, the waves keep on keepin' on, as if the device and the object under it were never there.

Because of this behavior, anything that hid under this device would go undetected by traditional sonar.

"Invisibility cloaks" have been in the news since 2006, when scientists proposed that they could design synthetic materials that would bend light in ways that made the objects under them appear invisible. Three years ago, Steve Cummer at Duke University figured out that the same principle could be applied to sound waves, and his lab has now brought out their first physical proof of the idea.

"Fundamentally, in terms of hiding objects, it's the same — how anything is sensed is with some kind of wave and you either hear or see the effect of it," Cummer told BBC News. "But when it comes to building the materials, things are very different between acoustics and electromagnetics."

The first-ever sound cloak, inspired by Cummer's 2008 proposal, was built at the University of Illinois, Urbana-Champaign, in January this year. That design works for inaudible ultrasound frequencies that are traveling under water. But Cummer's new device works for sounds that are traveling in air, in the frequency range between one and four kilohertz. (This corresponds to the last two octaves on the high end of the piano, BBC News explains.)

In addition to shielding ships from sonar, the new structure could be used to coat walls and soundproof rooms. With some fine-tuning to the design, the device could also be used to enhance the acoustics of concert halls.